1. Field of the Invention
The present invention relates to a method for optical coating. More specifically, the present invention relates to a method for compensating for radial variations in the coating rate of substrates on a rotating platen.
2. Related Art
Mirrors are used in ring laser gyroscopes to reflect two counter propagating laser waves in a laser cavity. The mirrors are made on a glass substrate coated with oxide layers. Each oxide layer has a specific thickness designed to meet spectral or other dielectric requirements. The oxide layers coated on each mirror alternate between an oxide layer having a high index of refraction and an oxide layer having a low index of refraction. For example, titanium dioxide (TiO2) is used for the high index oxide layers and silicon dioxide (SiO2) is used for the low oxide index layers.
The oxide layers are coated on the mirrors using ion beam sputtering (IBS). The mirrors are placed on a platen under a target plate containing the oxide or metal to be sputtered to form the oxides on the mirror. An ion beam gun bombards the target plate with argon ions, which breakaway oxide molecules or metal atoms from the target plate. The metal atoms form oxides in an oxygen filled background in the vacuum chamber. The oxide molecules then coat the surface of the substrates on the platen.
A problem with IBS is that the flux of oxide molecules from the target plate is non-uniform, resulting in non-uniform coating of the mirrors. The flux pattern is usually characterized by a peak flux with the flux dropping off in a radial direction from the peak flux. The non-uniform coating of the mirror is undesirable in ring laser gyroscopes because it causes distortions in the laser wavefront.
A conventional method to reduce the non-uniformity in the coating uses planetary motion of the mirrors. The mirrors are arranged in concentric circular rows on substrate platens, which rotate on a main platen. During coating, the center of the main platen rotates while the substrate platens rotate on the main platen. This causes the mirrors to trace a double rotating pattern, which tends to average out the position of each mirror. This results in positional independence of the mirrors and coating uniformity.
A problem with the planetary motion method is that it requires rotating the mirrors about two platen axis, which drives up the cost of the coating apparatus. In addition, the planetary motion involves moving the mirrors over a wide area. To cover the wide area, the target plate has to be positioned farther away from the platens to widen the traverse extent (diameter) of the flux area. This reduces the flux, which decreases with 1/distance2, where the distance is between the target plate and the platens. The reduction in the flux results in a reduction in the coating rate of the mirrors, thereby increasing the coating time of the mirrors.
Another method to reduce the non-uniformity in the coating is to rotate the mirrors about one platen axis. The mirrors are arranged in concentric circular rows on a platen. During coating, the platen rotates, which tends to average out the angular position of the mirrors with respect to the platen axis. This results in angular positional independence of the mirrors and coating uniformity in the angular direction. This method is more cost effective than the planetary motion method because the mirrors only rotate about one platen axis. In addition, the mirrors move over a smaller area compared to the planetary method, allowing the target plate to be positioned closer to the platen. This increases the flux and hence the coating rate of the mirrors.
A major problem with the single axis rotation method, however, is that it does not reduce the non-uniformity in the coating in the radial direction. In addition, positioning the target plate closer to the platen causes the flux pattern to drop off more rapidly from the peak flux, resulting in even greater non-uniformity in the radial direction.
Therefore, there is a need for a mirror coating method that reduces the non-uniformity of the coating in both the angular and radial direction, while still maintaining the cost effectiveness and high coating rate of the single axis rotation method.
The present invention provides a quick and easy method for a mask shape to reduce coating non-uniformity in the radial direction. A test run of the ion beam sputtering system coats a stationary glass plate having the same area and shape as the platen. The resulting coating thickness is measured across the surface of the glass plate and plotted as a function of position. This plot is the plot of the flux distribution. The coating rate pattern is determined by coating thickness by coating time. The desired width of the mask, which takes the form of a segment of a radial band, is adjusted to obtain the desired average coating rate for the entire segment, masked and unmasked. This is repeated for each radial band. All these radial widths put together design the functional form of the mask, the use of which provides a uniform coating of the platen in a radial direction.